JPH0814568B2 - Carbon dioxide sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a solid electrolyte having a structure using an ionic conductor used for measuring or controlling the carbon dioxide concentration in applications such as facility horticulture, environmental hygiene, disaster prevention, and medical care. Type carbon dioxide sensor.

2. Description of the Related Art In recent years, there has been an increasing need for a carbon dioxide gas sensor mainly in the fields of air conditioning and agriculture and livestock, and various types of sensors have been developed and put into practical use. Among them, the solid electrolyte type carbon dioxide gas sensor is superior to the infrared absorption type carbon dioxide gas sensor in many points in terms of durability, compactness and cost. However, since the sensor structure is complicated, many man-hours are used in the manufacturing process, and many problems remain in manufacturing.

An example of the above-described conventional carbon dioxide gas sensor will be described below with reference to FIGS. 7 and 8.

7 and 8 show the structure of a conventional solid electrolyte type carbon dioxide gas sensor. In FIG. 7, a thin plate-shaped ion conductive ceramics pair 11 is a thin plate-shaped NASICON plate 11a using a sodium ion conductive ceramics composed of a solid electrolyte and a thin plate-shaped yttria stabilized zirconia YSZ which is an oxygen ion conductive ceramics. It is composed of a plate 11b and both are solid-phase bonded at about 1300 ° C. NASI
The composition of CON is Na _{1 + X} Zr ₂ Si _X P _3-X O ₁₂ , (0 ≦ X ≦ 3). Further, this ion conductive ceramics pair 11 is provided with a pair of porous electrode layers 12 and 13 at both ends, and sodium carbonate 14 is attached to a part or all of the one electrode layer 12. Here, the side to which the sodium carbonate 14 is attached is referred to as the cathode layer 12, and the side without it is referred to as the anode layer 13. In addition, this negative / anode layer has lead wires for extracting voltage signals.
15a and 15b are attached by bonding portions 16a and 16b, respectively. Further, the gas sensing part 1 composed of the above-mentioned parts of FIG. 8 is provided with a heater 2 for heating on one side lower part, and is drawn out from the pins 5c, 5b connected to the lead wires 15a, 15b and the heater 2 for heating. Pin 5 connected to the two lead wires 5a and 5b
It is fixed by penetrating the lower pedestal 4 via c and 5d.
The lead wires 15a and 15b and the lead wires 5a and 5b are for signal output and voltage application, respectively.

The protector 3 is made of stainless steel wire mesh to protect the enclosed gas sensing unit 1, heater 2, lead wires 15a, 15b, 5a, 5b from mechanical damage and to improve contact with the measurement atmosphere. It is fixed to.

In the above configuration, the gas sensing unit 1 constitutes the following battery. Ie When the gas sensing unit 1 constituting the above battery is heated to the measurement temperature by the heater 2, the following battery reactions occur at the interfaces between the batteries. That is, Interface 2Na ⁺ = 2Na ⁺ (in NASICON) ………… Interface 2Na ⁺ (in NASICON) + O ^2- (inYSZ) = Na ₂ O …………
… The overall cell reaction of this entire cell is summarized below.

That is, all-cell reaction Na ₂ CO ₃ = Na ₂ O + CO ₂ (5) Therefore, an electromotive force represented by the following equation is generated between the negative and positive layers.

Here, E: electromotive force, P ^* : total atmospheric pressure R: gas constant, ^P CO ₂ : carbon dioxide partial pressure in atmosphere F: Faraday constant, ^a Na ₂ O: activity of Na ₂ O in NASICON From the activity formula of T: absolute temperature, ^a Na ₂ CO ₃ : Na ₂ CO ₃ , the generated electromotive force E is proportional to the logarithm of the carbon dioxide partial pressure in the atmosphere. Therefore, this electromotive force is applied to the cathode layer.
12, The carbon dioxide concentration in the atmosphere was electrically detected as a voltage signal by taking out from the anode layer 13 via the lead wires 15a and 15b.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, the above configuration has the following problems.

The first problem was that it was difficult to join the NASICON board and the YSZ board and to secure the adhesive strength.

That is, when these two plates are joined together, a high temperature of 1200 ° C. or higher is required, so that an intermediate product with a loss of ionic conductivity is easily produced at the interface, and the above battery reaction is not established.

The second problem is that the structure of the sensor is complicated, the number of manufacturing steps is increased, and it is difficult to put the sensor into practical use at low cost.

The third problem is that since the detection principle of this sensor is the equilibrium voltage measurement, the response is slow, so much time is used for measurement.

The present invention solves the above problems, and an object of the present invention is to provide a carbon dioxide gas sensor having a simple structure, easy manufacture, and excellent responsiveness.

Means for Solving the Problems A carbon dioxide gas sensor according to a first means for achieving this object is an ion conductive ceramics plate and an oxygen ion conductive ceramics, which have conductivity with respect to metal ions that can form a metal carbonate. The plate and the both plates are bonded with a porous material having electron conductivity to form a bonding layer, and a pair of negative / anode layers having electron conductivity are provided at both ends thereof to form an ion conductive ceramic pair. The electrode layer on the oxygen ion conductive ceramics plate side of the ion conductive ceramics pair is used as an anode layer, and the ion conductive ceramics plate side is conductive to metal ions that can form the metal carbonate. The electrode layer is used as a cathode layer, and a metal-carbon salt layer that forms a dissociation equilibrium with carbon dioxide in a shape that covers a part or all of the cathode layer is formed. The shadow of
A gas blocking layer is provided to cover a part or all of the remaining portion other than the surface on which the anode layer is formed, and a pair of lead wires is bonded to the negative / anode layer, and the gas sensing section. And a heating unit for heating the measurement temperature to a measurement temperature.

A second means for achieving this object is the above-mentioned means, in which an ion conductive ceramics pair having conductivity with respect to metal ions capable of forming a metal carbonate is used as an ion conductive ceramics plate and an oxygen ion conductive material. And a mixed conductive ceramics plate having electronic conductivity, and a bonding layer in which both plates are bonded, and a part of the lead wire is embedded in the mixed conductive ceramics plate, and the mixed conductive ceramics plate is an anode. It is a layer.

A third means for achieving this object is that in the second means, a material having the same composition as that of the mixed conductive ceramic plate is mixed with the bonding layer forming the ion conductive ceramic pair.

A fourth means for achieving this object is the above-mentioned third means, wherein a substance having the same composition as that of the ion conductive ceramics plate is added to the metal carbonate layer, and a lead wire is embedded in the metal carbonate layer. The metal carbonate layer is used as a cathode layer.

A fifth means for achieving this object is the above-mentioned fourth means, wherein the wire diameter of the tip portion of the pair of lead wires is enlarged and deformed, and the enlarged and deformed tip portion is embedded in the negative / anode layer. It was made.

By the first means, the bonding layer is made to have electronic conductivity while avoiding direct contact between the ion conductive ceramic plate and the oxygen ion conductive ceramic plate having conductivity with respect to metal ions capable of forming metal carbonate. The use of the porous material of 1) can prevent the generation of an intermediate product having no ion conductivity, and the following battery reaction can proceed to detect carbon dioxide gas. That is, By replacing the oxygen ion conductive ceramics plate with the mixed conductive ceramics plate by the second means, the mixed conductive ceramics plate can also serve as the anode layer. By embedding the lead wire in this plate, the electrode can be formed. One layer is omitted.

By the third means, oxygen ion conductivity can be imparted to the bonding layer by adding a substance having the same composition as that of the mixed conductive ceramic plate to the bonding layer, and the following anodic reaction can smoothly proceed. It is possible and the responsiveness becomes faster.
That is, By the fourth means, since the substance having the same composition as that of the ion conductive ceramic plate is uniformly dispersed in the metal carbonate, diffusion of metal ions generated by the following cathode reaction into the ion conductive ceramic layer is promoted, The responsiveness is improved. Furthermore, since the lead wire is embedded in the metal carbonate layer, the metal carbonate layer also serves as the cathode layer and the electrode layer is omitted.

By the means shown in FIG. 5, the lead wires are expanded and deformed at their tips to be embedded in the mixed conductive ceramics plate and the metal carbonate layer, respectively, so that the adhesive strength of one lead wire can be improved.

Example Hereinafter, a first example of the present invention will be described with reference to FIG.

FIG. 1 shows the structure of a gas sensing portion of a carbon dioxide gas sensor according to a first embodiment of the present invention. The same parts as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted. In FIG. 1, a bonding layer 17 between a thin NASICON plate 11a made of sodium ion conductive ceramics and a thin yttria-stabilized zirconia YSZ plate 11b made of oxygen ion conductive ceramics is a metal containing an inorganic binder. With a series of conductive paste, insert it between the NASICON plate 11a and the YSZ plate 11b, 150 ℃
After drying for 15 minutes at 900 ℃, bake for 15 minutes, NASICON board 11a and Y
The SZ plate 11b is joined. The composition of NASICON is Na _{1 + X} Zr ₂ Si
_x P _3−X O ₁₂ , (0 ≦ X ≦ 3). Also, the gas barrier layer 18
Is composed of low melting point glass-based insulation paste,
After drying for 0 minutes, baking at 550 ° C for 10 minutes to bond layer 17 and NASICON plate 1
The joint surface between 1a and the YSZ layer 11b is covered so as to be shielded from the measurement atmosphere.

The operation of the carbon dioxide sensor configured as described above will be described below.

In the above structure, when the gas sensing portion 1 is heated to the measurement temperature by the heater 2 shown in FIG. 8, sodium carbonate 14 dissociates and equilibrates with carbon dioxide gas in the measurement atmosphere, and the following reaction occurs at the interface with the cathode layer 13. , Sodium ions Na ⁺ generated according to the carbon dioxide concentration in the atmosphere diffuse into the bonding layer 17 in the NASICON plate 11a. On the other hand, oxygen in the atmosphere in contact with the anode layer 12 is
The following reactions occur at the interface of O ₂ + 2e ⁻ ＝ O ^{2 − and the} generated oxygen ions diffuse into the bonding layer 17 in the YSZ plate 11b and further into the pores in the bonding layer 17 to form the NASICON plate 1
After reaching the interface between 1a and the bonding layer 17, the following battery reaction occurs.

2Na ⁺ + O ² − = Na ₂ O Therefore, in the gas sensing unit 1, the whole battery reaction 5 is established as in the conventional example. Therefore, as in the conventional example, the electromotive force proportional to the logarithm of the carbon dioxide concentration in the measurement atmosphere is negative.
The carbon dioxide concentration in the measurement atmosphere can be measured by electrically detecting the electromotive force generated in the anode layer.

Further, since the bonding layer 17 is shielded from the judgment atmosphere by the gas blocking layer 18, the oxygen partial pressure in the bonding layer is kept constant with the oxygen partial pressure in the atmosphere, and the interference of water vapor or the like in the measurement atmosphere. Not affected by gas.

As described above, by interposing the bonding layer composed of the conductive paste on the ion conductive ceramic pair having conductivity with respect to the metal ions capable of forming the metal carbonate, the bonding can be performed only at a high temperature of 1200 ° C or higher. It was possible to prevent the formation of intermediate products that interfere with ionic conduction at the joint surface between the ion conductive ceramic layer and the oxygen ion conductive ceramic layer at a relatively low temperature of 900 ° C. A carbon dioxide sensor that is easy to manufacture can be provided.

A second embodiment of the present invention will be described below with reference to FIG. FIG. 2 is a sectional view showing the structure of the gas sensing portion of the carbon dioxide sensor according to the second embodiment of the present invention. In addition,
The same parts as those of the first embodiment of the present invention are designated by the same reference numerals and the description thereof will be omitted. In Fig. 2, thin plate NASICON
The mixed conductive ceramics plate 19 that is solid-phase bonded to the opposite surface of the bonding layer 17 of the plate 11a has a mixed conductivity having a small temperature dependence of its electron conductivity in the measurement temperature range, that is, 400 ° C to 600 ° C. Ceramics, ie La _0.5 S
A mixture of r _0.5 CoO ₃ 35 mol% + SrTiO ₃ 65 mol% is formed into a plate shape and is formed into a dense fired body that is fired at about 1200 ° C. Further, the mixed conductive ceramic plate 19 is formed of Au.
The lead wire 20a is embedded.

The operation of the carbon dioxide gas sensor configured as described above will be described, but the description is omitted because it is the same as that of the first embodiment of the present invention.

As described above, instead of the oxygen ion conductive ceramic plate, it is possible to double as the anode layer by using the mixed conductive ceramic layer having both oxygen ion conductivity and electron conductivity, and the sensor configuration is simplified, A carbon dioxide sensor that is easy to manufacture can be provided.

The third embodiment of the present invention will be described below with reference to FIG. FIG. 3 is a sectional view showing the structure of a gas sensing portion of a carbon dioxide gas sensor according to a third embodiment of the present invention. The same parts as those in the second embodiment of the present invention are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 3, the joining layer 21 joining the thin NASICON plate 11a and the mixed conductive ceramics plate 19 is composed of Au-based conductive paste containing an organic binder and mixed conductive ceramics La _0.5 Sr _0.5 CoO ₃ powder. With 2-3 mol% added, NASI
It is formed by being interposed between the CON plate 11a and the mixed conductive ceramic plate 19, dried at 150 ° C. for 15 minutes, and then baked at 900 ° C. for 10 minutes to be fixed.

The operation of the carbon dioxide gas sensor configured as described above is described, but the details are omitted because it is basically the same as the second embodiment of the present invention.

In the above structure, since the ceramic powder having the same composition as the mixed conductive ceramic plate 19 is uniformly dispersed in the bonding layer 21, the oxygen ions diffused in the mixed conductive ceramic plate are oxygen in the bonding layer. Oxygen ions are diffused between mixed conductive ceramic particles without gas diffusion,
Reaching the interface between the NASICON plate 11a and the bonding layer 21 and reacting with the sodium ions that have diffused in the NASICON plate, the reaction of all cells proceeds rapidly without gas diffusion of oxygen in the bonding layer.

As described above, by adding the ceramic powder having the same composition as the mixed conductive ceramic layer to the bonding layer, the oxygen ion diffusion rate in the bonding layer can be improved, and the responsiveness of the sensor can be improved. Even if an organic binder is used for the bonding layer instead of the low melting point glass-based binder reactive with the ceramic layer, a chemically stable and strong bonding layer can be formed.

Hereinafter, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 4 is a sectional view showing the configuration of the gas sensing portion of the carbon dioxide sensor according to the fourth embodiment of the present invention. In addition,
The same parts as those in the third embodiment of the present invention are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 4, the thin NASICON plate 11a is in contact with the end surface opposite to the bonding layer 21. The metal carbonate layer 22 is a saturated solution of sodium carbonate containing 20 wt.
%, Added in paste form to one end of a NASICON plate, embedded with lead wire 20b, dried at 150 ° C. for 30 minutes and fixed.

The operation of the carbon dioxide sensor configured as described above will be described. However, since it is basically the same as that of the third embodiment of the present invention, its details are omitted.

In the above structure, sodium ions generated by the dissociation equilibrium reaction between carbon dioxide in the atmosphere and sodium carbonate in the metal carbonate layer 22 are uniformly dispersed in the metal carbonate layer 22, and diffuse between NASICON grains. Because of the battery reaction However, the reaction proceeds uniformly throughout the metal carbonate layer 22 and the reaction rate is improved.

As described above, by adding the ceramic powder having the same composition as the ion conductive ceramic layer to the metal carbonate layer, the diffusion rate of sodium ions in the metal carbonate layer is improved and the response speed of the sensor is improved. be able to.

A fifth embodiment of the present invention will be described below with reference to FIGS. 5 and 6. FIG. 5 shows the fifth aspect of the present invention.
2 is a cross-sectional view showing the configuration of a gas sensing part of a carbon dioxide gas sensor according to one embodiment. The same parts as those in the fourth embodiment of the present invention are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 5, the lead wires 23a and 23b are made of 0.1φ Au wire, and the tip ends are enlarged and deformed as shown in the enlarged view of the lead wire in FIG. 22, and embedded in the mixed conductive ceramic plate 19.

The operation of the carbon dioxide gas sensor configured as described above will be described, but the description thereof will be omitted because it is similar to the fourth embodiment of the present invention.

As described above, by deforming the tip of the lead wire, the contact area with the layer in which the lead wire is embedded is increased, the electrical contact resistance is reduced, and the adhesive strength is improved.

Effects of the Invention As described above, according to the present invention, the following effects can be obtained.

First, by interposing a bonding layer made of an electronically conductive conductive paste on the pair of ion conductive ceramics, an ion conductive ceramic plate that could only be processed at a high temperature of 1200 ° C or higher, and The bonding of the oxygen ion conductive ceramic plates can be performed at a relatively low temperature of 900 ° C, and it is possible to avoid the formation of intermediate products that interfere with the ionic conduction on the bonding surfaces, making it easy to manufacture carbon dioxide. A gas sensor can be obtained.

Secondly, by using a mixed conductive ceramics plate having both oxygen ion conductivity and electronic conductivity instead of the oxygen ion conductive ceramic plate, it can also serve as the anode layer and the sensor structure can be simplified. .

Third, by adding a ceramic powder having the same composition as that of the mixed conductive ceramic plate to the bonding layer, the diffusion rate of oxygen ions in the bonding layer is improved and the responsiveness is improved.

Fourthly, instead of the oxygen ion conductive ceramic plate, a mixed conductive ceramic plate having both oxygen ion conductivity and electron conductivity is used, and the mixed conductive ceramic plate also serves as the anode layer and the bonding layer. Since the bonding layer is formed by adding the ceramic powder having the same composition as the above, and the ion conductive ceramic powder is added to the metal carbonate layer, the two electrode layers of the conventional example can be omitted and the structure is simplified. At the same time, a carbon dioxide sensor having excellent responsiveness can be obtained.

In addition, in addition to the above-mentioned configuration, the fifth is that the tip portion in which the lead wire is embedded is enlarged and deformed, so that the adhesiveness with the embedded layer is improved and it can also serve as a metal electrode. It is possible to obtain a carbon dioxide sensor having a simple structure and high mechanical strength and easy to manufacture.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing the configuration of a gas sensing portion of a carbon dioxide gas sensor according to a first embodiment of the present invention, and FIG. 2 is a configuration of a gas sensing portion of a carbon dioxide gas sensor according to a second embodiment of the present invention. FIG. 3 is a vertical sectional view showing a gas sensing portion of a carbon dioxide sensor according to a third embodiment of the present invention, and FIG. 4 is a gas of a carbon dioxide sensor according to a fourth embodiment of the present invention. FIG. 5 is a vertical cross-sectional view showing the sensing unit, FIG. 5 is a vertical cross-sectional view showing the gas sensing unit of the carbon dioxide sensor according to the fifth embodiment of the present invention, and FIG. 6 is a carbon dioxide sensor according to the fifth embodiment of the present invention. Fig. 7 is an enlarged view of the tip of the lead wire connected to the negative and positive electrodes of the gas sensing part of Fig. 7, Fig. 7 is a vertical cross-sectional view showing the gas sensing part of the conventional carbon dioxide gas sensor, and Fig. 8 is a longitudinal section showing the structure of the conventional carbon dioxide gas sensor. It is a side view. 1 ... Gas detector, 2 ... Heater, 3 ... Protector,
4 ... Pedestal, 5a, 5b ... Lead wire, 11 ... Ion conductive ceramic pair, 11a ... NASICON plate, 11b ... YSZ plate, 12
...... Cathode layer, 13 …… Anode layer, 14 …… Sodium carbonate, 15
a, 15b …… Lead wire, 16a, 16b …… Bonding part, 17…
… Bonding layer, 18 …… Gas barrier layer, 19 …… Mixed conductive ceramic plate, 20a, 20b …… Lead wire, 21 …… Bonding layer, 22 ……
Metal carbonate layer, 23a, 23b ... Lead wire.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location G01N 27/46 ZAB (72) Inventor Takeshi Takeda 1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (72) Inventor Satoshi Sekido 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (5)

[Claims] 1. An ion-conducting ceramics plate having conductivity to metal ions that can form a metal carbonate, an oxygen-ion conducting ceramics plate, and both plates are joined with a material having a porous electron conductivity. To form a bonding layer, and a pair of electrode layers having electronic conductivity are provided on both sides of the bonding layer to form an ion conductive ceramic pair, and the electrode layer on the oxygen ion conductive ceramic plate side of the ion conductive ceramic pair is formed. As an anode layer, the electrode layer on the side of the ion conductive ceramics plate having conductivity to metal ions that can form the metal carbonate as a cathode layer, and a carbon dioxide gas in a shape that covers a part or all of the cathode layer. A metal carbonate layer that forms dissociation equilibrium is formed, and gas is blocked on the surface that covers part or all of the remaining part of the ion conductive ceramic pair other than the negative / anode layer forming surface. Wherein the gas sensing portion which is formed by joining a pair of lead wires to the negative-anode layer, the carbon dioxide sensor comprising a heating unit for heating the gas sensing portion to the measured temperature is provided with the. 2. An ion-conducting ceramics plate having an ion-conducting ceramics pair having conductivity with respect to metal ions capable of forming a metal carbonate, and a mixed-conducting ceramics plate having oxygen ion conductivity and electronic conductivity. 2. The carbonic acid according to claim 1, wherein the carbon dioxide is composed of a bonding layer formed by bonding both plates, and part of the lead wire is embedded in the mixed conductive ceramics plate to form the mixed conductive ceramics plate as an anode layer. Gas sensor. 3. The carbon dioxide gas sensor according to claim 2, wherein a material having the same composition as that of the mixed conductive ceramics plate is mixed in the bonding layer forming the pair of ion conductive ceramics. 4. A metal carbonate layer containing a substance having the same composition as that of the ion conductive ceramics plate, and a lead wire being embedded in the metal carbonate layer, wherein the metal carbonate layer serves as a cathode layer. The carbon dioxide sensor according to item 3. 5. The carbon dioxide gas sensor according to claim 4, wherein the pair of lead wires has a wire diameter of a tip portion enlarged and deformed, and the enlarged and deformed tip portion is embedded in the negative / anode layer.